1. Field of the Invention
The present invention primarily relates to a coil, a method of construction of said coil and an imaging device equipped with a coil of this type.
The chief object of the invention is the construction of coils which are capable of compensating for the parasitic effects of a magnetic field on charged particles by generating a magnetic field at a desired location.
1. Description of the Prior Art
Many types of equipment utilize beams of charged particles such as electrons, for example. It is a known practice to make use of electron beams in display devices such as, for example, image intensifier tubes, television cameras, display cathode-ray tubes or electron microscopes.
However, charged particles such as electrons, for example, are deflected by electric and/or magnetic fields. If fields of this type are not controlled, they induce image distortions. Any visual display device is subjected at least to the terrestrial magnetic field.
It is known that attempts have already been made to eliminate the influence of the terrestrial magnetic field on the image obtained by providing a shield with a view to guiding the magnetic fields. However, this solution proves unsatisfactory inasmuch as there does not exist any efficient magnetic shielding material which is also transparent. Thus it is not possible to place an efficient shield in front of a television camera lens or the screen of a cathode-ray display tube without reducing the intensity of transmitted light to an excessive degree.
The device in accordance with the present invention compensates for the influence of the terrestrial magnetic field on the formed image by generating a magnetic field which has substantially the same intensity as the disturbing magnetic field and opposite polarization.
Compensating magnetic fields are generated by means of a coil comprising a support forming a plate of substantially constant thickness. On this support is deposited a conductor which constitutes the coil. The coil in accordance with the present invention is intended to be placed on the path of the electromagnetic radiations to be displayed and/or used for display. To this end, it is essential that the disturbance of the electromagnetic waves supplied by the coil should in any case be of lesser significance than the inconvenience caused by the distortions produced by the magnetic field. The support is chosen so as to ensure minimum absorption of electromagnetic radiations which form part of the pass-band of radiations to be displayed and/or used for display. In all cases, this absorption must be uniform over the entire surface of the coil which intercepts said radiation. For example, if the electromagnetic radiation forms part of the visible spectrum, it will be an advantage to make use of a support of glass or of plexiglas. Plastic materials will be employed, for example, in the case of radiation forming part of the x-rays. In a particularly advantageous alternative embodiment, a material marketed under the trade name KAPTON by the Dupont de Nemours Company can be employed.
The conductive tracks deposited on the support form patterns which serve to produce the desired magnetic field when they are supplied with electric current.
In a first example of construction of the device in accordance with the present invention, absorption due to the patterns of conductors is negligible since the conductor can be considered as transparent in view of the thickness of the tracks employed. In the case of electromagnetic radiation which forms part of the visible spectrum, the conductors employed are transparent to light such as, for example, those employed in certain photovoltaic panels or in transparent computers. In the event that the electromagnetic radiation forms part of the x-rays, it is possible for example to use beryllium or aluminum deposited in a thin film, or plastic conductors.
In a second example of construction of a coil in accordance with the present invention, use is made of conductors in which absorption of electromagnetic radiations is liable to disturb the image. In such a case, provision is made for patterns having substantially uniform absorption over substantially the entire surface of the coil. By way of example and in order to achieve this objective, a uniform layer of conductors is deposited, subject to the resolution of the display device. For example, conductive tracks are constructed by forming cut-out portions in the uniform surface of the conductor, for example by chemical ablation. The effect of said cut-out portions will be to delimit conductive tracks. However, these cut-out portions are too fine to have a detectable influence on the formed image.
In a particularly advantageous alternative embodiment of the device in accordance with the present invention, use is made of a plurality of superposed conductive patterns separated by insulating layers. Thus it is possible to form complementary patterns in which the total absorption by the radiation (x-rays, for example) is substantially constant over the entire surface of the coil. This accordingly prevents any parasitic spatial and temporal modulation of the signal to be transmitted.
It is clearly possible to associate the coil in accordance with the present invention with other means for limiting the effects of parasitic magnetic fields. For example, the faces of the imaging device through which the electromagnetic radiation is not intended to pass are covered with shielding material which guides the magnetic field lines.